Nonaqueous electrolyte secondary batteries such as a lithium secondary battery are currently in wide use as (i) batteries for devices such as a personal computer, a mobile telephone, and a portable information terminal or (ii) on-vehicle batteries.
A device equipped with a lithium ion battery includes a wide variety of electrical protection circuits, provided in a battery charger or a battery pack, so that the battery operates normally and safely. For example, if a breakdown or a malfunction occurs in the protection circuits, the lithium ion battery may be continuously recharged. This may cause oxidative and reductive degradation of an electrolyte on surfaces of a positive electrode and a negative electrode both of which generate heat, oxygen release caused by decomposition of a positive electrode active material, and even deposition of metallic lithium on the negative electrode. This may eventually cause the lithium ion battery to fall into a thermal runaway. There is also a danger that ignition or explosion may occur in the lithium ion battery, depending on the situation.
In order to safely stop a battery before such a dangerous thermal runaway occurs, most lithium ion batteries currently include, as a separator, a porous base material which contains a polyolefin as a main component and which has a shutdown function of clogging pores present in the porous base material when a temperature inside the battery is raised due to some defect and reaches approximately 130° C. to 140° C. Exhibition of the shutdown function at a temperature rise inside the battery stops passage of ions in the separator, and thus allows the battery to safely stop.
As a porous base material useful as a nonaqueous electrolyte secondary battery separator, Patent Literature 1 discloses, for example, a polyethylene microporous film which has a flexion rate, a porosity, and an average pore diameter each falling within a specific range so that (i) a film thickness and a porosity necessary for development of strength are maintained and (ii) high ion permeability is achieved.